This invention relates in general to thermal insulation materials and, more specifically, to polyimide foam insulation having improved flame resistance.
Thermal insulation for pipes, ducts and the like is often made from foamed materials which have excellent resistance to heat flow. A wide variety of foamed plastics have been used for different applications. Typical of these are the polystyrene foam panels described by Charpentier in U.S. Pat. No. 3,863,908 phenolic foam panels described by Burning et al. in U.S. Pat. No. 3,885,010, and the polyurethane foam panels described by Willy in U.S. Pat. No. 3,298,884. However, many of these materials are flammable and give off toxic gases when exposed to flames. These materials are not acceptable for use in aircraft, ships, spacecraft, etc.
Polyimide foams have been found to be excellent insulating materials and to be flame resistant. Polyimides also do not emit toxic gases when exposed to direct flames. Polyimide foams are ordinarily produced in closed molds where specific thicknesses and surface configurations are required. Typical of such methods are those disclosed by Long et al. in U.S. Pat. No. 4,621,015 and Shulman et al. in U.S. Pat. No. 4,647,487.
Foam produced in closed molds tends to be very irregular, lacking uniformity of cell size, density and strength across the mold. Often, the gases emitted by the polyimide precursor during foaming inhibit the growth of cells in the foam, making foaming action low and unpredictable. These problems increase when attempts are made to vary the foam product density by varying the amount of precursor placed in the mold. In some cases, polyimide precursors are foamed at ambient pressure in open molds. The resulting foam block is then sliced parallel to the mold bottom to produce sheets and eliminate the rind that forms on the foam surface. While this produces more uniform cell size and density, it is very difficult to produce foams of selected different densities.
Recently, considerable interest has developed in using plastic pipe for its improved corrosion resistance and light weight, especially in U.S. Navy ships. Often, such pipes are made from fiberglass reinforced epoxy resins. Typical pipes are offered by the Ameron Corporation under the Bondstrand trademark. Since these pipes lack the heat and flame resistance of metal pipes, it is essential that maximum possible flame resistance be provided through insulation.
One standard requires that such insulated pipes withstand a flame temperature of 1600.degree. F. for a period of 15 minutes and must limit the temperature of the plastic pipe to no greater than 300.degree. F. Insulation is tested by placing the insulated pipe in a test cell with hexane fuel floated on water below the pipe. The hexane is ignited and luminous flames heat the test article above.
Most plastic foam insulation rapidly fails during that test, often giving off toxic gases and actually igniting. Some inorganic insulation materials can pass that test. However, those materials tend to be very heavy and difficult to install and often containing undesirable ingredients such as asbestos. While polyimide foams do not emit toxic gases and have good flame resistance, prior polyimide foams scorch badly and have seriously degraded surfaces when subjected to the above test. Also, prior flame resistant foams did not have sufficient rigidity to permit use as unsupported ducts or the like.
Therefore, there is a continuing need for improved polyimide foam insulation having improved flame resistance and which can be conveniently formed into pipe and duct insulation having selected variable physical characteristics.